1. Field of the Invention
Aspects of the present invention relate to a method of fabricating a polycrystalline silicon layer, a thin film transistor (TFT) fabricated using the same, a method of fabricating the TFT, and an organic light emitting diode (OLED) display device having the same. More particularly, aspects of the present invention relate to a method of fabricating a polycrystalline silicon layer, crystallized using crystallization-induced metal, wherein the method removes the crystallization-induced metal existing in a region of the polycrystalline silicon layer to be a channel by forming and annealing a metal layer or a silicide layer thereof. Aspects of the present invention further relate to a TFT having a semiconductor layer fabricated using the polycrystalline silicon layer formed by the method so as to significantly reduce a leakage current, a method of fabricating the TFT, and an OLED display device having the same.
2. Description of the Related Art
Generally, polycrystalline silicon layers have been widely used as semiconductor layers for TFTs because they have a high field-effect mobility and can be applied to high speed circuits and constitute CMOS circuits. TFTs using polycrystalline silicon layers are typically used as active elements of active-matrix liquid crystal display (AMLCD) devices and switching and driving elements of OLEDs.
Methods of crystallizing an amorphous silicon layer into a polycrystalline silicon layer include solid phase crystallization (SPC), excimer laser crystallization (ELC), metal induced crystallization (MIC) and metal induced lateral crystallization (MILC). SPC is a method of annealing an amorphous silicon layer for several to several tens of hours at a temperature at or below the transition temperature of the glass used as a substrate of a display device employing a thin film transistor (typically, about 700° C. or less). ELC is a method of crystallizing an amorphous silicon layer by irradiating the amorphous silicon layer with an excimer laser and locally heating the amorphous silicon layer to a high temperature for very short time. MIC is a method of using phase transfer induction from amorphous silicon to polysilicon by contacting is the amorphous silicon layer with metals such as nickel (Ni), palladium (Pd), gold (Au), and aluminum (Al), or implanting such metals into the amorphous silicon layer. MILC is a method of inducing sequential crystallization of an amorphous silicon layer by lateral diffusion of silicide formed by reacting metal with silicon.
However, SPC has disadvantages of a long processing time and a risk of transformation of the substrate due to the long processing time and high temperature used for the annealing. ELC has disadvantages in that expensive laser equipment is required and interfacial characteristics between a semiconductor layer and a gate insulating layer may be poor due to protrusions generated on the created polycrystallized surface. MIC and MILC have disadvantages in that a large amount of crystallization-inducing metal remains on the crystallized polycrystalline silicon layer to increase the leakage current of a semiconductor layer of a TFT.
Recently, methods of crystallizing an amorphous silicon layer using metal have been widely being studied in order to develop a way to crystallize the amorphous silicon layer in a shorter time and at a lower temperature than SPC. The crystallization methods using metal include MIC and MILC, discussed above, and super grain silicon (SGS) crystallization, discussed below. However, methods of crystallization using a crystallization-inducing metal have a problem that characteristics of the TFT may deteriorate due to contamination by the crystallization-inducing metal.
Thus, after the amorphous silicon layer is crystallized using a crystallization-inducing metal, a gettering process may be performed to remove the crystallization-inducing metal. The gettering process is generally performed using an impurity such as phosphorous or a noble gas, or by a method of forming an amorphous silicon layer on a polycrystalline silicon layer. However, even by these methods, the crystallization-inducing metals may not be is effectively removed from the polycrystalline silicon layer, and high leakage current may still be a problem.